Filter driers are used in air conditioning and refrigeration systems to remove water and other impurities from the oils and refrigerant materials which are used as working fluids in such systems. Filter driers, which are also called receiver dehydrators, are generally of two types. One type includes a solid core desiccant material which serves as filtration media. Solid cores of desiccant are often made from molded alumina or other materials having similar filtering and absorption properties. An example of a filter drier that has a solid desiccant core is shown in U.S. Pat. No. 4,908,132 which is owned by the Assignee of the present invention.
Another type of filter drier used in refrigeration and air conditioning systems uses a desiccant material that is in particle form. Care must be exercised in the construction of such filter driers to insure that the desiccant is contained within the body of the unit. If particles of desiccant material flow out of the filter drier and enter other components of the system, damage may occur.
Prior art filter driers have used a number of approaches to contain the particles of desiccant material. Some prior art filter driers have held the particles of desiccant between a pair of perforated plates inside the body of the unit. Usually pads of fiberglass or other fiberous material are positioned between the desiccant particles and the plates to aid in containment. The use of fiberous pads helps to avoid extrusion of the desiccant particles through the perforations in the plates which would otherwise occur when the plates are pressed together to pack the desiccant particles in relatively tight relation. Such packing is desirable from a filtration standpoint with many types of desiccants.
Another approach to containing desiccant particles is disclosed in U.S. Pat. No. 4,436,623 which is owned by Multiform Desiccants, Inc. This patent has the desiccant particles encapsulated in a cartridge made of polyester fabric. An advantage of this construction is that it simplifies assembly of the filter drier. This construction enables the loose desiccant to be assembled into the cartridge at a remote location. Upon assembly of the filter drier, the cartridge is simply placed in the housing. This approach makes assembly much easier and reduces the chance that particles will work loose during assembly and will later flow out of the filter drier causing damage to the remainder of the system.
The filter drier construction shown in U.S. Pat. No. 4,436,623 has several serious shortcomings, however. A fundamental problem is that it tends to become clogged with contaminants. This occurs because the fine polyester fabric and compressed desiccant at the upstream end of the cartridge readily becomes saturated with dirt and water. Once the fabric and desiccant at the upstream end of the cartridge are saturated, further impurities tend to be held at the surface of the cartridge. These impurities cannot reach the underlying desiccant which is available to absorb them. As a result, refrigerant flow through the cartridge is restricted and the filter drier must be replaced.
The configuration of the desiccant cartridge shown in U.S. Pat. No. 4,436,623 is also susceptible to being blocked by amounts of contaminants that may be readily handled by other types of filter driers. As a result, use of this construction is not practical in most applications. Further, the cartridge configuration shown in this patent is relatively expensive. This is because a plastic pipe must be incorporated into the cartridge so an outlet tube may extend through the center of the filter drier. This adds cost, both in terms of material and assembly time.
There is also a need to vary the amount of desiccant used in filter driers. Different systems may use different desiccant materials which may necessitate a desiccant bed of greater or lesser thickness. Some systems may require more or less desiccant because of system cleanliness requirements, or a greater anticipated level of contamination. Different refrigerants may also necessitate the use of more or less desiccant material in the filter drier.
Applicant is aware of no filter drier construction in which varying amounts and types of desiccant material may be readily incorporated without the use of customized parts that are made for the particular filter drier design.
Thus, there exists the need for a filter drier that provides ease of assembly, superior contaminant holding capability and low cost modular construction.